Polyisocyanate, used as a raw material of polyurethane, is industrially produced, for example, by allowing polyamine to react with carbonyl chloride for isocyanate reaction.
In such a producing of polyisocyanate, after completion of the isocyanate reaction, high-molecular-weight polyisocyanate, which is byproducts produced as residues are heated and concentrated by distillation operation thereby to be separated from the crude polyisocyanate (cf. Patent Document 1, for example).
Meanwhile, it is known that the crude polyisocyanate containing polyisocyanate residues is undesirably subject to thermal polymerization, such as dimer reaction, trimer reaction, and carbodiimide reaction, by the application of heat. In the distillation operation, while polyisocyanate is distillated together with a Cl-containing gas secondarily produced, such as hydrogen chloride and carbonyl chloride, the polyisocyanate residues are concentrated. While being concentrated, the polyisocyanate residues are thermally polymerized as described above and increased in viscosity, to cause unstable transport of the residues and further a possible blockage of the transport line.
Meanwhile, an evaporation operation using a thin film evaporator is known as a refining technique for an organic compound susceptible to thermal polymerization and is widely used industrially.
Further, in recent years, there have been proposed methods wherein the residues produced is continuously fed to a reactor vessel in its liquid state or solution state, while also high-temperature and high-pressure water is continuously fed to the reaction vessel, with temperature of the reaction vessel controlled to 190-300° C., whereby the residues are decomposed to polyamine and the polyamine is recovered (cf. Patent Document 2, for example).
[Patent Document 1] Japanese Unexamined Patent Publication No. 2002-518369.
[Patent Document 2] Japanese Unexamined Patent Publication No. Hei 10-279539.